1. Field of the Invention
This invention relates to a process of making sheet material comprising a substrate web of synthetic thermoplastic material and an adhesive layer consisting of a non-crosslinkable elastomer.
2. Description of the Prior Art
Process of making such webs are known from Published German Application No. 25 27 942. In one of said known processes the non-crosslinkable elastomeric material is fed, heated and plasticized by means of a screw extruder and is then fed through a fitting to a slot die having a distributing passage. In the form of a flat film, a web of non-crosslinkable elastomeric is then applied to the substrate web of thermoplastic material. In another embodiment of that process, a two-layer slot die is used to make a co-extruded flat web. Another apparatus disclosed in said published application is used for a two-layer film blowing process, in which the plasticized elastomeric material for the adhesive layer and the thermoplastic material for the substrate web are supplied to a two-layer film blowing head. In all said processes the elastomeric material is processed and fed by an extruder.
It is known that variations in the output rate of extruders can be reduced by means of spinning pump type gear pumps, e.g., in the extrusion of layers of thermoplastic layers.
It is known to use devolatilizing extruders in the processing of elastomeric material. As a rule, elastomeric materials contain special components consisting of oils and other tackiness-improving additives, which when heated to elevated temperatures during the further processing are undesirably volatilized so that bubbles and holes are formed in the finished product. To prevent such volatilization, the elastomeric material is heated above its devolatilization temperature in a first part of the devolatilizing extruder and is then transferred through a grooved throttling member into a cylinder part provided with a vent opening. In that cylinder part the feed channel of the feed screw is so deep that the elastomeric material to be devolatilized can fill the channel only to one half of its depth. As a result, there is a large free surface on the screw over a length which is at least 5 times the diameter of the screw and the volatile constituents can be sucked from said free surface through the vent opening by means of an applied vacuum.
In the second part of the extruder the elastomeric material which has been devolatilized must be recompressed and subjected to a pressure which will overcome the backpressure applied by the extrusion die. This is effected in that the depth of the channel of the devolatilizing screw is reduced.
But in the extrusion of an insulating adhesive film the plastic material must be subjected to a pressure that is about ten times higher than in the processing of rubber. Such a high pressure cannot be applied unless the screw is sufficiently increased in length. In that case the elastomeric material to be compressed will be heated to such a high temperature that a second devolatilization could not be avoided.
That undesired heating could be avoided by the use of a shorter feed screw in the second part of the extruder but the backpressure applied by the extrusion die could not be overcome in that case. As a result, part of the elastomeric material to be fed would be retained and would flood the vent opening and enter the deep channels of the devolatilizing part of the feed screw. A reliable devolatilization would be prevented even if only the devolatilizing channels were entirely filled.
In the processing of a coextruded insulating sheet consisting of a polyethylene substrate web and a coextruded adhesive layer made of a non-crosslinkable elastomer, substantially butyl rubber, it has been found that the backpressure applied by the film blowing head was so high that the vent opening was flooded under all operating conditions so that a devolatilization was prevented.